Optically pumped semiconductor lasers that emit at a wavelength of 2 to 5 microns or longer have typically been preferably pumped by light that has a wavelength close to and slightly shorter than the wavelength emitted by the optically pumped laser. In a typical optically pumped laser (OPL) each photon of the pump light can generate at most one electron-hole pair, by exciting an electron (the electrical charge carrier) from the valence band into the conduction band. The excited electron then can make an optical transition back to the valence band (an interband transition that also can be described as having the electron recombine with a hole) to produce at most one photon of the light emitted by the OPL. If the energy of the pump photon is much higher than the energy of the photon emitted by the OPL, the excess pump energy typically is not used effectively. Instead, the excess pump energy, which is related to the difference in the wavelengths of the pump light and the light produced by the optically pumped laser (OPL), is typically converted to heat. This heat must be removed. Otherwise, the temperature of the optically pumped laser can increase and the non-radiative processes, such as Auger recombination, that occur in the optically pumped laser can reduce the efficiency of that laser or can prevent that laser from operating at high output powers.
The more efficient semiconductor pump lasers emit at shorter wavelengths such as 0.98, 1.48 and 1.55 microns. These wavelengths can be quite different from the wavelength of the OPL. Thus, those OPL that are pumped by these more efficient pump lasers could be subject to more heating. It is desirable to make more efficient use of the pump energy so that less heat is produced in the OPL that are pumped by shorter-wavelength light.
One way to improve the efficiency of the laser is to use an energy cascade that enables one electron injected into the cascade to undergo multiple transitions, thereby generating multiple photons. Although some prior lasers have a cascade of interband or intraband wells, these prior art lasers require application of an external electrical voltage to establish the cascade (that voltage being supplied with the electrical pumping).
Electrically pumped cascade lasers that employ interband transitions for emission of the light are described in U.S. Pat. Nos. 5,799,026 and 6,404,791, with an extensive review of those lasers given in IEEE Journal of Quantum Electronics, v.38, n.6, pp. 559-568 (2002). Electrically pumped cascade lasers that employ intraband cascade laser with wide emission spectrum is described in Nature, v.415, pp. 883-887 (2002). In these prior art lasers, the electrons and holes that recombine to produce the emitted light are supplied from an external source of electrical current, such as a battery or a power supply. These prior lasers have to be electrically pumped to achieve the cascade, since it is the externally applied voltage accompanying the externally supplied current that establishes the electric field across the structure that forms the cascade of light-emitting regions and carrier injection regions in these prior lasers. Unlike these prior art electrically pumped lasers, the present specification discloses optically pumped lasers that are not externally biased to achieve the cascade.
Optically pumped lasers can potentially be more efficient than electrically pumped lasers. Part of this greater efficiency is achieved because the OPL does not need to have any p-doped layers. The free-carrier absorption that especially occurs in the p-doped layers of electrically pumped lasers is an undesirable loss mechanism. The amount of free-carrier absorption can be much lower in the optically pumped lasers.
Optically pumped lasers with integrated pump-light absorbing layers are described in paper CMM4 of the Digest of 2000 Conference on Lasers and ElectroOptics (CLEO), pages 63-64 and in Journal of Applied Physics, v.92, n.10, pp. 5621-5626 (2002). Unlike these prior art optically pumped lasers, the present specification discloses OPL containing integrated pump-light absorbers that have tilted valence and conduction bands. Such tilted bands would have no benefit in these prior art lasers and would unnecessarily complicate their construction.
Antimonide-based lasers that have light-emitting regions comprising W-shaped potential well structures with staggered, Type-II band alignments are described in Applied Physics Letters, v.67, n.6, pp. 757-759 (1995) and in Journal of Applied Physics, v.89, n.6, pp. 32883-3289 (2001). To improve the lasing efficiency, it is preferred in these prior art lasers that the W-shaped wells in them all be the same. Some embodiments of the present specification likewise make use of W-shaped potential wells. In contrast to the prior art, however, the presently disclosed Type II antimonide lasers implement cascades comprising multiple W-shaped potential wells that are not the same. According to the present disclosure, at least two different kinds of W-wells are implemented in each cascade.